NiCad (nickel-cadmium) battery technology has been employed successfully in portable hand-held applications for many years. Photographic equipment, power tools, data terminals, personal radio transceivers, and pagers commonly utilize NiCad batteries as a power source. The charging systems that have been provided with these products have ranged from a simple transformer/rectifier type to rather complex systems to monitor and control the charging function. An increasing need is the ability to charge NiCad batteries quickly. To reliably and efficiently charge NiCad batteries at high rates requires careful control of the charging operation to avoid damage to the cells, particularly under extreme ambient temperature conditions.
The NiCad charge cycle consists of two basic parts: the coulombic portion and the overcharge portion. The coulombic portion of the charge cycle is characterized by the fact that most of the charge that is applied to the battery is stored in the form of electrochemical energy. This portion of the charge cycle is slightly endothermic, consequently high charge currents may be applied during this time without resulting in temperature increase. Most of the available battery capacity is stored during the coulombic portion of the charge cycle. The overcharge portion of the charge cycle is characterized by the fact that most of the applied charge current causes generation of oxygen gas at the positive electrode of the NiCad cell, with only a relatively small amount of charge actually being stored in the cell. The released oxygen chemically recombines with cadmium at the negative electrode of the cell which serves to equalize the internal pressure of the cell. If the overcharge rate is too high, the rate of oxygen recombination may be insufficient to prevent excessive internal pressure and cell venting, which drastically reduces the useful life of the cell.
The most critical factors in determining the maximum allowable charge current that may be safely applied to a NiCad battery are temperature and state of charge. At low temperatures the oxygen recombination rate is significantly reduced which limits the allowable overcharge current that may be applied without venting the cells if they are fully charged. At high temperatures the heat released by the oxygen recombination reaction may cause excessive cell temperature to be experienced leading to premature failure of the plate separator material and subsequent short-circuiting.
If the battery is fully discharged, minimal oxygen generation will occur until the battery nears the fully charged condition. If the battery is nearly fully charged, it will quickly enter the overcharge condition and begin oxygen generation. The difficulty lies in accurate determination of the previous state of charge to avoid damage to the battery.
Additionally, battery chargers sometimes use a constant-current source to charge batteries. If a varying load is placed in parallel with a battery under charge, the current delivered to the battery may not be sufficient to charge it in a specified time. If the load is too great, the battery may actually be discharged. This is particularly true if the constant current source producing the charging current does not have the capability of adequately servicing both battery recharging and varying load, or with a varying load is too unpredictable in magnitude or duration to insure it will not adversely affect battery recharging.
One solution to this problem is to provide a current source which is either variable or without question can produce enough current to handle all situations regarding battery charging and varying load. However, a significant problem with these types of current source is the possibility of excessive heat dissipation which can adversely affect either the recharging process, operation of the device, or both.
Still further, there is room for improvement in recharging batteries of the type being discussed. A variety of factors can interfere with the effective and efficient charging of batteries. An improvement in the flexibility and efficiency of recharging would be desirable in the art.